Patent Application
20110288224
The present invention relates to nanocomposite materials based on a polymeric or plastic matrix and a layered silicate (clay). The nanocomposites prepared have improved mechanical properties (e.g. rigidity, resistance to breakage), improved thermal properties (e.g. greater thermal stability) and improved gas and vapour barrier properties (e.g. to oxygen, water vapour, aromas) and do not require clay-matrix compatibilizing agents. They also have by default a barrier to electromagnetic radiation in UV, Vis and IR and fire resistance, maintaining high transparency levels and they use substances permitted for food, pharmaceutical and biomedical contact. The application of these nanocomposites is multi-sectorial, such as, for example, their advantageous application both in the packaging of products of interest for food and for applications in other sectors.
In this same sense, the present invention relates to several processes for the preparation of these same nanocomposite materials.
The nanocomposite materials once prepared can be transformed into the end product by any transformation process of plastics, such as and without limitation, moulding by blowing, by injection, by extrusion or by thermoforming.
At present, polymer/clay nanocomposites based on polyamide 6 (PA6) are marketed for applications related to the automotive industry or high-barrier packaging. Polyamide 6 is a semicrystalline thermoplastic that has good mechanical resistance, tensile strength and high resistance to impact; it has good behaviour to sliding, improved with the MoS2 aggregate, it also has good resistance to wear; for this reason it is suitable as an engineering plastic of universal use, in mechanical constructions and industrial maintenance work.
Nanocomposites based on polyolefins are receiving special attention in the research world due to the wide range of uses of this type of polymers, as well as the good properties of these materials, mainly their low cost, good processing and recycling capacity. Conventional compounds (microcompounds) of several polyolefins are already used in industry, but the addition of a low content of charges dispersed in the polymer with at least a dimension in the order of nanometres makes it possible to achieve an improvement in the end properties of the material (nanocomposite) which is impossible to obtain with conventional charges. Thus, it can improve properties such as the mechanical, thermal or gas barrier properties.
In recent years, the interest for the exfoliation of clays of smectite type (such as montmorillonite) in all type of polymeric matrices has grown exponentially due, on the one hand, to the great aspect ratio of the layers forming this type of layered silicates and, on the other, to the great availability of this type of charge compared to others. In general, clays in their natural state (without modifications) have a highly hydrophilic character and are only miscible with very hydrophilic matrices such as polyethylene oxides (PEO) or polyvinyl alcohol (PVOH). On many occasions, mixing with some polymers gives rise to a completely immiscible system. Therefore, these clays have been modified with surfactants (such as ammonium salts) to make them more similar to the polymeric matrices. Furthermore, this modification manages to increase the interlayer spacing (basal) of the clay. Therefore, the size of the surfactant chains is of great influence in obtaining a greater or lesser basal spacing on modifying the clay. But in polyolefin matrices this is not sufficient to achieve a good exfoliation of the layers of clay by melt mixing.
In the case of polyolefins, such as polypropylene and polyethylene, their non-polar nature means that there is no affinity between the polymeric chains (with hydrophobic character) and the clay (with hydrophilic character). To improve this compatibility problem, many methods have been proposed in recent years. One of them is to functionalize the polymeric matrix with functional polar groups via the use of a catalyst. Another possibility would be the addition of a percentage of compatibilizer (already functionalized polyolefin) to the polymer and clay system, (e.g. Morawiec et al. Eur. Pol. J. 2005, 41, 1115). In recent years, some authors (López-Quintanilla et al.; Journal of Applied Polymer Science, 2006, 100, 6, 4748) have used the route of dispersing clay in a matrix that is 100% functionalized with polar groups to obtain a masterbatch with a high clay content and later dilute it by melt mixing it with polyolefin. These three methods have been employed mainly with the use of clays of smectite type. These clays have been previously treated with surfactants or other organic substances for the surface modification of the clay and to make it hydrophobic (character more similar to polyolefins), ammonium salts being the modifier most employed up to now. These methods, in addition to making the end product more expensive due to the use of compatibilizers (more expensive than the polyolefin matrix), may have incompatibilities with some applications of the end materials as is the case of food packaging, since substances are used to modify the clays that are not approved for food contact.
The availability of functionalized polyolefins is highly limited due to the chemical difficulties that arise during functionalization. Many studies based on polyolefin nanocomposites have used a polyolefin functionalized with maleic anhydride (MA) as this is the most available to date. But these polyolefins functionalized with MA have a complicated molecular structure as a consequence of the impurities that may remain and from the lateral reactions, including the degradation of the chains that occur during the grafting process of the free radicals. Hence, other authors have sought alternative ways of compatibilizing the polyolefin matrix with the clay.
For example, patent US20050014905A1 proposes the use of functionalized polyolefins with hydrophilic terminal functional groups to be able to even exfoliate clay in its natural state (which has not been treated with organic surfactants or other acids) and to be able to later mix this batch with pure polyolefin conserving the exfoliated structure resulting from mixing the clay with the functionalized matrix.
Patent US 006864308B2 has used intercalating agents (esters of hydroxyl substituted carboxylic acids and hydroxyl substituted amides), which are solid at ambient temperature, to treat clays of the smectite group (modified or not modified with organic salts) before melt mixing them with a polyolefin.
Patent US 005910523A has treated nanometric charges with aminosilanes before being mixed with a maleated or carboxylated polyolefin matrix, with the object of favouring the interactions between the functionalized surface of the clay and the carboxyl or maleate groups. The mixing of the charge and the maleated polyolefin matrix has been performed by the dissolution of both components in xylene at 120° C.
Patent US 006838508B2 discloses a new process to modify clays of smectite type such as the intercalation of a quaternary ammonium ion with at least a terminal reactive group and a —Si—O—Si group. Here it discloses how, after the modification, the clay is mixed with a polyolefin matrix compatibilized with maleic anhydride by the dissolution of both components in toluene in high temperature conditions during a certain time. This batch obtained by dissolution is later mixed with polyolefin to obtain the final nanocomposite.
In short, it can be observed that until now, in studies referring to nanocomposites of polyethylene or other polyolefins, modifications have been used of the clay with quaternary ammonium salts of the type wherefrom several hydrocarbons hang which in many cases are not substances allowed to be in contact with food, and therefore, they are not suitable for applications in food packaging. Likewise, they are also salts which undergo heat degradation at temperatures below those used during the processing to transform the nanocomposite into the end product. On the other hand, other methods proposed by some authors are based on the use of toxic solvents (toluene, xylene, etc) to dissolve polymer plus clay and thus optimize the interactions between both components and maximize the degree of improvement in the final properties of the nanocomposite (generally mechanical), but these methods are contaminating from an environmental and health standpoint, whilst being prohibited for certain final applications. Together with these factors, the use of compatibilizers or of functionalized polymeric matrices does not make the product cheaper and at times harms the properties of the pure material, and the preparation of polyolefin nanocomposites using two steps (two melting processes, a melt masterbatch plus later melt processing, etc.) involves a greater consumption of the energy resources than those processed, for example, in a single step.
Some authors have used solvents such as water to prepare nanocomposites by extrusion. In the study by Kato et al. (Pol. Engineering and Sci. 2004, 44, 7, 1205), water is injected during the extrusion of polypropylene nanocomposites with unmodified montmorillonite clay but, together with the supply of polypropylene, they add 30% by weight of compatibilizer (polypropylene functionalized with maleic anhydride) and a small quantity of a quaternary ammonium salt. Therefore, the exfoliation and the improvements in the properties observed cannot be associated with an effect of water injection in one of the sections of the extruder. Patent US 006350805B1 discloses a method to prepare nanocomposites of polyamide-montmorillonite by melt mixing from the use of solvents such as water to favour the dispersion of the clay and improve the mechanical properties as well as the heat dispersion temperature.
The present invention discloses nanocomposite materials with polymeric or plastic matrix and a layered silicate (clay). The nanocomposites prepared have improved mechanical properties (e.g. rigidity, resistance to breakage), improved thermal properties (e.g. greater thermal stability) and improved gas and vapour barrier properties (e.g. to oxygen). Furthermore, these nanocomposites also have a barrier to electromagnetic radiation, provide fire resistance and impact to a minimal extent transparency. Finally, they are composed of materials permitted by legislation for food, pharmaceutical and biomedical contact and do not require clay-matrix compatibilizing agents.
Therefore, a first essential aspect of the present invention relates to new nanocomposite materials that comprise at least the following:
a) A layered silicate.
b) A polymeric or plastic matrix.
In this same sense, the layered silicate is a clay selected from the group formed by the dioctahedral or trioactahedral family, of kaolinite, gibbsite, dickite, nacrite, halloysite, montmorillonite, micaceous, vermiculite or sepiolite nature, and more preferably of kaolinite nature. Preferably, the layered silicate is a clay from the family of type 1:1, which is composed of a tetrahedral layer of silicate (with a practically zero degree of substitution of silicon by other cations) bonded to a dioctahedral layer of gibbsite type. The chemical formula of this material is typically Al2Si2O5(OH)4.
The percentage of layered silicate in the nanocomposite in the clay polymer is from 0.05% to 98% by weight, the percentage being dependent on the desired final properties of the nanocomposite material. Typically, the percentage of clay is between 0.01% and 98% and, more preferably, from 0.05 to 40%.
With respect to the polymeric matrix, it can be selected from any type, thermoplastics, thermostable materials and elastomers such as polyolefins, polyesters, polyamides, polyimides, polyketones, polyisocyanates, polysulfones, styrenic plastics, phenol resins, amide resins, ureic resins, melamine resins, polyester resins, epoxydic resins, polycarbonates, polyvinyl pyrrolidones, epoxy resins, polyacrylates, rubbers and gums, polyurethanes, silicones, aramides, polybutadiene, polyisoprenes, polyacrylonitriles, PVDF, PVA, PVOH, EVOH, PVC, PVDC or derivatives of biomass and biodegradable materials such as proteins, polysaccharides, lipids and biopolyesters or mixtures of all of these and they may contain all types of additives typically added to plastics to improve their manufacturing and/or processing or their properties. Although they will more preferably be selected from the group of polyolefins, preferably of the type of high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), ultra high density polyethylenes, metallocene polyethylenes and particularly ultra low density polyethylenes, polypropylene (PP), ethylene copolymers, polyethylenes functionalized with polar groups and ionomers of ethylene or any combination thereof. Preferably, polyolefins will be selected from the type of high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and all their families of polypropylenes (PP) and their copolymers.
According to a preferred embodiment, the matrix may additionally incorporate agents or substances, including any nanoadditive or concentrate of nanoadditives thereof described in this document or of other nanoclays, with barrier properties to electromagnetic radiation, fire resistance or active and/or bioactive substances.
Typically, the percentage of polymeric matrix against the quantity of clay nanoadditive is between 2 and 99.9%, preferably between 60% and 99.9%.
These new nanocomposite materials are characterized in that they significantly improve properties such as rigidity and mechanical resistance, resistance to breakage or in the heat stability of the polymer base, as well as improvements in gas and vapour barrier properties, in the barrier to electromagnetic radiation and in fire resistance. All these improvements are due to the morphology obtained which is formed by a combination of structures (intercalation, exfoliation and aggregation) where the dispersed particles are in the order of a few nanometres.
A second essential aspect of the present invention relates to three different processes to obtain the same nanocomposite material, with the same properties and characteristics described above. These processes are:
1) Mixing of the components in solution which, without being limiting in nature, comprises the stages of:
In a later stage and starting from the concentrate of clay obtained by any of the three aforementioned manufacturing methods, the clay concentrate can be processed, for example, to obtain gravel, by any plastic manufacturing method together with additives typically used to formulate or process plastics or alternatively it can be added to the same or another plastic matrix by any plastic processing method and adding any additive typically added in the formulation or processing of plastics.
An alternative step that is also envisaged is the addition, to the matrix during any stage of the processes previously described, of any other additive, including any nanoadditive or concentrate of nanoadditives thereof described above or from other nanoclays, typically used as a barrier to electromagnetic radiation and/or of fire resistance and/or active and/or bioactive.
A last fundamental aspect of the present invention relates to the use of new nanocomposite materials in different sectors or for different applications due to their improved properties, as previously described and, without being limiting in nature, such as:
Below, the invention is described with reference to the attached figures, wherein:
All the above characteristics and advantages, as well as others typical of the invention, can be better understood with the following examples. On the other hand, the examples do not have a limiting character but are illustrative so that the present invention can be better understood.
The formation of nanocomposites formed by a low density polyethylene which incorporate a kaolinite clay has been performed (which until now had not previously been used by other authors in polyolefin nanocomposites). Said clay is added to the melt system dispersed in a polar solvent, typically water or alcohols. To highlight the novelty of the invention, the results obtained were compared with nanocomposites prepared from the incorporation of a clay of montmorillonite nature modified with ammonium salts and also those prepared with natural montmorillonite clay (without surface modification treatment). This comparison is important as the clay principally used in polyethylene (and also polypropylene) nanocomposites is of montmorillonite type.
After obtaining the nanocomposites formed by LDPE with 7% by weight of kaolinite (K) clay content (
A morphological analysis is carried out for each of the samples using scanning electron microscopy (SEM) and the differences were observed in the type of clay dispersion and the final aggregate size. On adding the unmodified montmorillonite clay dispersed in a solvent such as ethanol or water, a typical morphology is obtained of a microcompound (conventional compound) with large clay aggregates (formed by hundreds and thousands of clay layers) of several microns, up to aggregates of more than 20 μm. The montmorillonite clay modified with commercial salts (Cloisite®20A) achieves dispersion, partly, in small aggregates of large aspect ratio and in the size of the nanometres, but part of the clay remains aggregated, forming large tactoids of several microns. However, on adding the untreated kaolinite clay, object of this invention, a nanocomposite is obtained in suspension where all the particles are homogeneously dispersed in the order of nanometres in the form of small tactoids (formed by a reduced number of layers). Due to these differences in morphologies, positive differences are found in the nanocomposites prepared by the addition of kaolinite via liquid to the plastic.
As is observed in the mechanical results shown in table 1, a greater dispersion of the clay involves a greater increase in rigidity (elastic modulus E), in the yield stress (σy) and in the breaking strength of the sample (σrot), whilst not reducing to a large extent the tensile strength (εrot) of the material thanks to the small size of the particles. The heat stability (or start of the degradation of the nanocomposite, T0.1, which shows the temperature at which 10% of the initial material has degraded) is extraordinarily improved, by the addition of kaolinite in dispersion. Table 2 shows the values of the oxygen permeability, where it is also possible to observe how a greater dispersion of the clay leads to a greater reduction in gas permeability, in particular to oxygen.
| Number | Date | Country | Kind |
|---|---|---|---|
| P200802035 | Jul 2008 | ES | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/ES2009/070277 | 7/8/2009 | WO | 00 | 5/5/2011 |
